1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to a servo control for a disk drive, and more particularly, to a servo control for effective compensation of low frequency disturbance incoming to a disk drive.
2. Description of Related Art
In general, a disk drive is a data storage apparatus that reads or writes data from or to a disk by using a magnetic head. Because a disk drive tends to have a high capacity, a high density, and a compact size, a bit per inch (BPI) in a rotational direction and a track per inch (TPI) in a radial direction of the disk drive become large. As a result, there is a demand for a highly accurate mechanism in the disk drive.
An aspect of track following control of the disk drive is to locate a head at the center of a target track. Track following control error may occur due to various types of disturbance. In particular, a portable drive, such as a micro drive, is vulnerable to low frequency vibration and impact.
Referring to FIG. 1, a conventional servo control system for a disk drive includes a repetitive run-out (RRO) compensator 110, a state estimator 120, state feedback controllers 130A and 130B, summing units 140A and 140B, and a voice coil motor (VCM) driver/actuator 150.
Generally, a circuit block including the RRO compensator 110, the state estimator 120, the state feedback controllers 130A and 130B, and the summing units 140A and 140B are collectively referred to as a track following control circuit 1000. In the present invention, the track following control circuit 1000 is represented by a transfer function −C(z).
The summing units 160A and 160B are equivalent to disturbance d and disk run-out, respectively, occurring in a head disk assembly of the disk drive.
The state estimator 120 has a function of estimating values of position, speed, and bias from a position error signal (PES) and a control input. The RRO compensator 110 has a function of compensating for RRO disturbance, XRUNOUT, due to a disk eccentricity. The state feedback controllers 130A and 130B calculate a control input signal by combining the estimated values of position, speed, and bias and apply a control input signal to the VCM driver/actuator 150. The estimated value of bias is used for an integrator function in the control circuit to compensate for the low frequency disturbance component in the disturbance d incoming to the system. However, if a gain of the integrator is increased to improve low frequency disturbance compensation performance, there is a problem in that stability of the control system is lowered.